This invention relates to an improved solid core for a golf ball, golf balls made with the new cores, and a composition of matter suitable for molding such cores.
Two-piece golf balls consisting of a molded resilient core and a cover are now widely available. They consist of a peroxide cross-linked, mixed polybutadiene-metal acrylate molded core and a separately molded cover typically comprising an ionomer resin such as the type sold by E. I. Dupont de Nemours Company under the trademark SURLYN. The balls are extremely popular because they can be manufactured to be substantially cutproof and to travel further when hit than any other type of commercially available USGA regulation golf ball.
The distance a ball will travel when hit (hereinafter "distance") is a function of many factors including angle of trajectory and clubhead speed. Among the factors of concern to manufacturers which affect distance are the coefficient of restitution of the ball, and the ball's surface configuration, i.e., dimple pattern. Coefficient of restitution is a measurement familiar to those skilled in the golf ball art. One way to measure the coefficient is to propel a ball at a given speed against a hard massive surface, and measure its incoming and outgoing velocity electronically. The coefficient of restitution is the ratio of the outgoing velocity to incoming velocity.
The coefficient of restitution in solid core balls is a function of the composition of the molded core and of the cover. In so-called three-piece balls comprising a liquid or solid center, elastic thread windings, and a cover, the coefficient of the core is a function primarily of the composition of the elastomeric thread windings and center, and the winding tension.
Those seeking to increase the coefficient of restitution of golf balls are restrained effectively by the U.S. Golfers Association regulations which mandate that a "regulation" ball cannot have an initial velocity in excess of 255 feet per second. All other things being equal, as the coefficient of restitution of a ball is increased, the ball's initial velocity will also increase.
U.S. Pat. No. 3,819,768 to R. P. Molitor discloses that the coefficient of restitution of golf balls having any given core can be increased significantly by applying to the core a cover material comprising a blend of a sodium neutralized ionomer resin with a zinc neutralized ionomer resin. Over the years, distance in two-piece golf balls has been steadily improved by virtue of the development of embodiments of the Molitor invention which give further increases in coefficient and the development of improved molded cores.
Cores suitable for use in manufacturing two-piece balls are disclosed in U.S. Pat. Nos. 4,264,075, 4,169,599, 4,165,877, and 4,141,559. Currently, it is believed that the highest coefficient molded cores comprise a high cis content polybutadiene and a zinc salt of an alpha beta ethylinically unsaturated monocarboxylic acid, e.g., zinc di or mono acrylate or methacrylate, cured with conventional free radical initiator-type peroxide catalysts.